1. Field of the Invention
The present invention relates to a semiconductor memory device, such as a dynamic random access memory (DRAM), and in particular to a semiconductor memory device which can increase the sensitivity of a sense amplifier and which does not require the generation of a bit line reference voltage using a dummy cell.
2. Related Arts
A DRAM, one type of semiconductor memory device, generally has a memory cell consisting of one transistor and one capacitor, and information is recorded in the DRAM by accumulating an electric charge in the capacitor of the memory cell. A potential of the capacitor of the memory cell is read to a bit line, and a small difference between potentials generated at a bit line pair is detected and amplified by a sense amplifier.
For the conventional DRAM, mainly employed is a method for setting a precharge level at a bit line to half a potential of a power source on the high potential side. When the high potential side power source is Vcc, the precharge potential at the bit line is Vcc/2. And when the potential of the memory cell is high because an electric charge has been accumulated in the capacitor, the potential at the bit line is increased from the precharge potential by the equivalent of a minute voltage. When the potential of the memory cell is low because an electric charge has not been accumulated, the potential at the bit line is dropped from the precharge potential by the equivalent of the minute voltage. The potential at a bit line on the opposite side is maintained at the precharge level, and the minute voltage difference is generated between the two bit lines. The minute voltages difference is detected and amplified by a sense amplifier.
Since a precharge potential is set to half the potential at the high voltage side power source, there is no need for a circuit, such as a dummy cell, that generates a reference voltage on an opposite bit line.
However, in accordance with the recent low power consumption trend, the power voltage tends to be lower, or an internal reduced power voltage lower than the external power voltages tends to be internally generated and employed. When half the potential of the power source is employed as a bit line precharge potential, therefore, a problem arises in that it is difficult to drive an amplification transistor in a sense amplifier. That is, the sense amplifier generally serves as a differential circuit comprising a pair of transistors whose source terminals are connected in common, and even when half the potential of the power source is applied to the gate terminals of the transistors, a gate-source voltage tends to be lower than or near a threshold voltage of the transistors. As a result, the sensitivity of the sense amplifier is degraded, or in the worst case, the sense amplifier can not be driven.
Instead of the method wherein half the potential of the power source is used as a precharge potential, proposed is a method for setting a precharge potential at a bit line to a ground potential or to a potential available at a high potential side power source. According to this method, transistors in a sense amplifier can be adequately driven. When the precharge potential at a bit line is set to a ground potential or to a potential available at a high potential power source, a circuit to generate a reference potential at a opposite bit line is necessary. This circuit is, for example, a dummy cell, as is described above. The dummy cell method is not preferable because it requires an extra circuit and consumes extra power when driving the dummy cell. Although another method is proposed wherein a third potential is generated and is employed as a reference potential, it is very difficult to provide a circuit for stably generating such an intermediate potential.